Metal sheets having apertures (such as holes, gaps, slits, slots or other openings) have many decorative and utilitarian uses. One utilitarian use is as a metal mask. Such metal masks are often used as templates for selectively exposing a material or workpiece to various manufacturing operations. For example, they may be used to screen conductive metal paste material onto a substrate to form desired features, such as conductive lines between electrical components of a circuit card. They may also be used to control the areas machined during laser ablation operations. Metal masks may also be used in photolithography operations where only certain areas of a photoresist are to be exposed. The features defined by such masks, particularly those used in integrated circuit fabrication, are often very small and must be defined with a high degree of precision. As the drive towards denser circuits continues, so does the need for masks capable of precisely defining very small and closely spaced features. Close spacing of the features, which results in less mask material between them, makes mask strength an increasingly important mask characteristic.
It should be understood that while the present invention is particularly directed to forming metal masks, the present invention is also generally applicable to the forming of apertures in metallic material for many decorative and utilitarian uses.
One way of making metal masks stronger is by increasing the thickness of the metal material used to form the mask. However, conventional methods of making masks generally lose their precision when applied to material thicknesses which are significantly greater than the smallest feature to be defined.
It would be desirable to be able to form metal masks and other metallic objects with apertures that are smaller in size than the thickness of the metallic material.
In general, it would also be desirable to have an improved process for forming apertures in metallic sheets.
In photolithography, one conventional means for forming metal masks, a photoresist is applied to a metal sheet and the photoresist is then exposed and developed to define features. Thereafter, the metal sheet with the photoresist layer is etched so as to replicate the feature defined by the photoresist in the metal sheet. However, features having dimensions significantly smaller than the thickness of the metal (i.e., an aspect ratio greater than 1) cannot be formed due to the isotropic nature of the etching portion of the process. That is, the etchant continues to remove material in the lateral direction until it has penetrated the thickness of the material. This lateral material removal also causes the rounding of inside corners of mask features, resulting in inside corners having a radii of no less than half the thickness of the mask material, thereby making square corners impossible to obtain by conventional photolithographic methods. The ability to form sharp corners on thick metallic masks is highly desirable (i.e., wire bond pads thus have more area).
LaPlante et al., U.S. Pat. No. 5,168,454, the disclosure of which is incorporated by reference herein, discloses a laser drilling technique employing a laser in the 3-10 W (average power) range which is used to machine apertures as small as 0.5 mils in a workpiece. However, this technique does not work well when applied to metal sheets having thicknesses as great as 25 mils. In the case of thick metal sheets, the material to be removed is not fully severed from the body of the metal sheet due to non-uniform penetration by the laser through the thickness of the metal and melting and resolidifying of the metallic material (i.e., rewelding) in the kerf area, thereby retaining the feature in the metal sheet. The rewelded material, and hence the feature, cannot be easily removed by mechanical operations such as punching or flexing without damaging the metal mask.
Increasing the power of the laser to the 100-200 W (average power) range would result in full penetration of thick metal sheets but also poor accuracy, poor edge definition, rounded corners, larger cut width, and possibly also warping of the metal sheets.
Howrilka et al., IBM Technical Disclosure Bulletin, 21, No. 3, p. 961 (August 1978), the disclosure of which is incorporated by reference herein, discloses the laser drilling of a blind hole in an epoxy substrate followed by etching in an acid to remove debris in the bottom of the hole. Since an epoxy substrate is drilled, there is no possibility of rewelding of the metallic feature to the adjacent metal sheet.
Melcher et al., U.S. Pat. No. 4,283,259, the disclosure of which is incorporated by reference herein, has disclosed maskless chemical and electrochemical machining wherein an energy source, such as a laser, is used to induce local heating in the workpiece while being simultaneously submerged in an etchant to speed up the chemical etching reaction and thereby preferentially remove material from the heated area. However, the thickness of the workpiece material must still be nearly as thin as the smallest dimension to be etched due to the isotropic nature of the etchant.
IBM Research Disclosure 26969, September 1986, p.572, the disclosure of which is incorporated by reference herein, discloses dry laser etching of a 1 mil wide slot in a 2 mil thick molybdenum mask. The laser, typically in the 5-10 W power range, penetrates the molybdenum by locally heating and oxidizing molybdenum to MoO.sub.3 which is volatile at the elevated temperature caused by the local heating. The volatilized MoO.sub.3 is carried away by a moving gas stream. Any recrystallized MoO.sub.3 that is deposited on the molybdenum mask may be removed mechanically or by dissolution in a solvent. This technique, however, does not lend itself to other metals, such as stainless steel, whose gas phases occur at or above their melting points. Nor would this technique be feasible for thick metal sheets since with thick sheets, it is difficult to provide enough heat to vaporize all the metal. And, even if vaporized, the vaporized metal would redeposit on the walls adjacent to the holes being drilled. Further, for thick material, all the material is not uniformly removed, leaving bridges of material within the kerf.
Others have proposed localized atmospheres to assist or promote the laser working process.
Yoshida et al., U.S. Pat. No. 5,187,148, the disclosure of which is incorporated by reference herein, discloses a sputtering method wherein a laser causes ablation of a target, thereby causing the generation of a laser plasma. Oxygen is supplied to the laser plasma.
Dulcy et al., U.S. Pat. No. 4,972,061, the disclosure of which is incorporated by reference herein, discloses the laser irradiating of a surface to roughen it, thereby generating a surface plasma. A localized atmosphere (nitrogen or oxygen) is provided at the surface to promote a chemical change at the surface.
While the prior art is replete with methods for the working of metals with a laser, there still remains a need for a process for forming small apertures with a laser in sheets (thick or thin) of metallic material.
Accordingly, it is a purpose of the present invention to have an improved process for forming small apertures in sheets of metallic material.
It is another purpose of the present invention to have an improved process for forming small apertures in sheets of metallic: material wherein a laser is used.
It is a further purpose of the present invention to have an improved process for forming small apertures with a laser in sheets of metallic material wherein the size of the aperture can be significantly smaller in dimensions than the thickness of the metallic sheet.
It is a further purpose of the present invention to have an improved process for forming square-cornered apertures with a laser in sheets of (thick) metallic material wherein the radii of the corners of the apertures can be significantly smaller in dimension than half the thickness of the metallic sheet.
These and other purposes of the present invention will become more apparent after referring to the following description considered in conjunction with the accompanying drawings.